We will complete our structural, including ultrastructural, and functional assessment of the hypoxic response in infant and adult rats and begin to assess the reversal of these changes on return to room air for periods up to three months. The ultrastructural studies will include quantitative point-counting techniques applied both to the precursor smooth muscle cells and to mature PVSM. Having developed a technique for measurement of oxygen consumption, we will compare cardiac output and pulmonary vascular resistance in hypoxia in the young and old rats. The response to acute hypoxia by the structurally altered pulmonary vascular bed will also be studied. Using immunofluorescent techniques, we will follow the development and nature of smooth muscle proteins in the developing smooth muscle cells, particularly of the nature of myosin and the appearance of tropomyosin. In addition, the unlabeled antibody Peroxidase Anti-Peroxidase method will be developed and utilized for recognition of these proteins by both light and electron microscopy.